"Our results suggest that these dietary supplements have little to no effect on fertility and may even cause mild gastrointestinal symptoms," researcher Enrique Schisterman said in a news release from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, part of the U.S. National Institutes of Health. He is with the institute's division of intramural population health research.

The researchers recruited more than 2,300 U.S. couples planning to get fertility treatment. They randomly assigned the men to take a pill containing zinc and folic acid or a placebo.

Births didn't differ between men who got the supplement and men who didn't. Among men who took the supplement, there were 404 births, compared with 416 among men who took the placebo, the researchers reported.

Sperm health was also similar between the groups. But the proportion of broken DNA in the sperm was higher in the supplement group (30%) than in the placebo group (27%). Studies have linked a high rate of broken sperm DNA with infertility, the researchers noted.

Men who took the supplement also reported more abdominal discomfort, nausea and vomiting than men who took the placebo.

The report was published Jan. 7 in the Journal of the American Medical Association.